1. Field of the Invention
The present invention relate to an optical modulation element and a projection apparatus using the optical modulation element.
2. Related Background Art
Conventionally, a reflection diffraction grating type optical modulation element as shown in the perspective view of FIG. 3 has been proposed.
FIG. 3 shows a light modulator (optical modulation element) whose surface (reflecting surface) three-dimensionally deforms and has strip-shaped elements 31 each made of a thin film in a one-dimensional array on a substrate 30. Four strip-shaped elements 31 correspond to one picture element. The strip-shaped elements 31 are provided in a number corresponding to the picture elements in the horizontal direction.
The substrate 30 can be formed using, e.g., silicon. A conductive layer 34 made of, e.g., tungsten or a tungsten alloy is formed on the substrate via a passivation layer (not shown) made of, e.g., silicon nitride (Si3N4). Each strip-shaped element 31 can be formed from, e.g., silicon nitride and has its two ends fixed on the substrate 30 at positions separated from conductive layer 34. An intermediate flat portion 32 is spaced apart from the substrate 30 while forming a gap 33. The upper surface of the flat portion 32 is coated with a light reflecting material such as an aluminum film.
The thickness of the gap 33 can be adjusted by changing the magnitude of a voltage to be impressed between the strip-shaped element 31 and the conductive layer 34. When no voltage is impressed, the strip-shaped element 31 projects, as shown in FIG. 4A. When an appropriate voltage is impressed between the strip-shaped element 31 and the conductive layer 34, the strip-shaped element 31 deforms due to electrostatic attraction, so the flat portion 32 of the strip-shaped element 31 bends toward the substrate 30 and comes into contact with the substrate 30, as shown in FIG. 4B.
Assume that all strip-shaped elements 31 project, as shown in FIG. 4A. When light having a wavelength λ becomes perpendicularly incident on the surface of the substrate 30, all reflected light components from the element flat portions 32 of the strip-shaped elements 31 are in phase, and four elements 31 which form one picture element functions as a flat mirror.
Assume that four strip-shaped elements 31 alternately exhibit projecting and recessed shapes, and for example, the height difference between the projecting portion and the recessed portion is λ/4. When light having the wavelength λ becomes perpendicularly incident on the surface of the substrate 30, the phase of reflected light from each protecting portion surface is shifted from that of reflected light from each recessed portion surface by λ/2, as shown in FIG. 6. Interference occurs between these reflected light components, so the reflected light emerges from the surface as reflected diffraction light having an angle θ with respect to a line perpendicular to the surface.
Hence, for example, when a lens for focusing light reflected at the angle θ is arranged, an arbitrary one of the plurality of picture elements can be displayed as a dark picture element by modulating four strip-shaped elements 31 corresponding to that picture element to the state shown in FIG. 5, and an arbitrary one of the plurality of picture elements can be displayed as a bright picture element upon receiving light by modulating four strip-shaped elements 31 corresponding to that picture element to the state shown in FIG. 6. FIG. 7 is a graph showing the impressed voltage vs. intensity of reflected light characteristic of a reflection diffraction grating having the above structure.
In the above prior art, however, a fixing portion 301 for supporting and fixing the effective reflecting portion 32 on the substrate 30 is necessary, as shown in FIG. 3 or 4A and 4B. Since the size of the fixing portion 301 is 20 μm or more, light incident on the fixing portion 301 causes flare. Additionally, in the above prior art, since the strip-shaped elements cannot be made compact in the longitudinal direction, the two-dimensional array of elements reduces the aperture ratio of the picture elements.